EP-A-242,208 describes a composite membrane for ultrafiltration purposes comprising a porous inorganic support which must meet special requirements in that the surface has a particular pore diameter, and is preferably of anodic aluminium oxide. These requirements make the membrane too expensive for most uses.
In EP-A-136,937, a process of producing a microporous membrane is proposed, wherein a metal oxide or metal hydroxide sol, obtained by hydrolysis of a metal compound, is thickened and acidified and subsequently applied on a supporting layer, followed by drying and eliminating the thickening agent, and sintering. Although this process is said to prevent the formation of cracks, the complexity of the process is a serious drawback; furthermore, the permeability of the membrane is undesirably low.
EP-A-40,282 describes a process of producing an ultrafiltration membrane by coating a porous support, whereby the porous support is treated with a volatile liquid such as acetone or methanol, which liquid serves to draw the coating material into the pores of the support.
For the same purpose, it is known to saturate the porous support with water prior to applying the microporous layer forming suspension.
According to EP-A-144,097, the penetration of microporous layer forming particles into the support is prevented by reducing the contact time between the microporous layer forming suspension and the support.
However, the membranes obtained by these known processes are not free from pin-holes, if the support contains pores of more than about 5 .mu.m. Also, the permeability of the membranes is insufficient.